1. Technical Field
This invention relates generally to a roller cone rock bit and, more particularly, to the structure of the gage inserts and rotatable cones of such a bit.
2. Background Information
One form of drill bit used in drilling a borehole in the surface of the earth is a roller cutter bit. In one type of roller cutter bit, the cutters each have a cone shaped body with a plurality of hard material cutting elements or inserts protruding from the surface of the body. As the bit is rotated under weight against the earth, the inserts penetrate rock in a gouging scraping action to chip away formation material and form a borehole.
In drilling a borehole, it is important that the drilled diameter of the hole or gage be maintained throughout the service life of the drill bit. One reason for this is simply that the borehole must be large enough to accommodate the next bit when the first one is replaced. If the hole is too small, the replacement bit may become unnecessarily worn before reaching the bottom of the hole. In a typical roller cone bit, two different types of inserts are used to cut and maintain the borehole wall at the gage diameter. In the art, one of these inserts is called the gage insert. On a roller cone body, a row of gage inserts, the gage row, is located at the base or heel of the cone so that when the cone is rotated, bottom hole cutting edges on the gage inserts cut the bottom of the borehole to the desired gage diameter. A second surface on the gage inserts acts against the borehole wall to maintain the gage diameter.
A roller cone bit having inserts of the forgoing type is disclosed in U.S. Pat. No. 2,774,570. Therein, inserts characterized as heel series inserts have a rounded or ovoid surface contour and are positioned on the body of the cone to bisect the angle formed by the two portions of the cone which act to cut the bottom and form the wall of the borehole. Thus, these heel series inserts act with rounded surfaces on both the bottom and wall of the borehole. Cooperating with the heel series inserts are inserts mounted in the gage cutting portion or gage wall of the cone body. These latter inserts, termed gage inserts in Pat. No. 2,774,570, are described as being rounded or ovoid in shape and forming a slight protrusion beyond the gage surface of the cone for disintegrating formation at the borehole wall and maintaining gage.
In an earlier patented form of roller cone bit, the use of separate inserts in the gage wall of the cone body was avoided. U.S. Pat. No. 2,687,875 discloses the orientation of a cylindrical insert having an ovoid or rounded cutting end in either of three different positions relative to the gage wall at its juncture with the bottom hole cutting portion of the roller cone body. In one position (FIG. 3), the insert bisects the angle between the two portions of the cone. In another orientation (FIG. 5), the ovoid end of the insert is disposed to act on the wall of the hole with a side surface of the insert positioned to contact and disintegrate the bottom of the borehole. In the third position (FIG. 4), the side of the insert extends parallel to the borehole wall and may be flattened to present a greater surface to effect disintegrating action at gage.
To improve the wearability of gage inserts in U.S. Pat. No. 4,058,177, an asymmetric gage row insert is disclosed as providing a larger amount of wall contacting surface. This larger surface acts to decrease the wear on the gage insert and increases the ability of the bit to maintain gage. Specifically, the asymmetric insert has a cylindrical base integrally joined with an asymmetrical head and telescoped into a socket in the body of the cone. The head projects from the surface of the cone body and includes a gage cutting surface which is the largest plane surface of the head. With respect to the surface of the cone body, this plane surface is angled to contact the borehole wall at substantially the gage angle of the bit when drilling. Thus, the gage cutting surface presents a relatively large flat surface for wearing against the borehole wall and maintaining the gage diameter.
While the roller cone bits of the foregoing type have resulted in improved maintenance of gage, changes in drilling techniques which require that the direction of drilling be changed while drilling have resulted in the need for roller cone drill bits which are easier to steer.